Fix typo in README (#384)

Author: matthewjnield

README.md

@@ -59,7 +59,7 @@ xml2 has similar goals to the XML package. The main differences are:
   free the memory used by an XML document as soon as the last reference
   to it goes away.
 
-* xml2 has a very simple class hierarchy so don't need to think about exactly 
+* xml2 has a very simple class hierarchy so you don't need to think about exactly 
   what type of object you have, xml2 will just do the right thing.
 
 * More convenient handling of namespaces in Xpath expressions - see `xml_ns()`